1. Field of the Invention
The present invention relates to an image reading apparatus which reads an image by means of an image pickup device and produces an image signal.
2. Description of the Prior Art
In an image reading apparatus for use in a digital copier, a facsimile system, an image file or the like, an original carrying an image, characters or any other type of information is irradiated with light from a rod-shaped light source. Reflected light from the original is received by a solid-state image sensor through an imaging lens so as to convert the original information into an electric signal. In an apparatus of this type, shading characteristics must be corrected. Shading characteristics are caused by: nonuniform illumination characteristics of the light source, contamination of a reflector, a decrease in the F-number of a lens toward its periphery in accordance with the so-called cos.sup.4 .theta. characteristics (where .theta. is the half field angle of the lens), and variations in sensitivity between bits of the image sensor. It has been proposed to perform such shading correction in accordance with a signal obtained upon reception of reflected light from a white board as a standard for shading correction.
Meanwhile, in an image reading apparatus of the type described above, it has also been proposed to arrange a plurality of image sensors along the scanning direction so as to improve resolution and transfer speed. In this case, an optically noncontinuous region may not be formed between regions which are scanned by the respective image sensors. Thus, overlap regions are formed at the peripheries of regions scanned by the respective image sensors, which overlap regions are scanned by more than one image sensor. A reference marker is placed in each such overlap region. In response to a signal corresponding to each such reference marker, outputs from the image sensors are linked together for continuity.
When such linking is performed in accordance with a signal corresponding to a reference marker, calculation of the linking amount may involve an error which is attributable to the shading characteristics mentioned above. Such an error must be eliminated.
When these means are actually adopted in an image reading apparatus, these means are preferably assembled in the apparatus with a simple construction and without mutual interference.